Mt-cdma using spreading codes with interference-free windows

ABSTRACT

The invention relates to digital transmission. It particularly relates to a method of transmitting data using multi-carrier Code-division Multiple Access (CDMA) for accessing a transmission system. The method comprises a step of modulating the data to be transmitted using Orthogonal Frequency-division Multiplexing (OFDM) for producing OFDM modulated data symbols and a step of spreading the OFDM modulated data symbols with spreading codes including a set of predefined sequences, wherein the sequences are predefined so that they satisfy predetermined auto-correlation and/or cross-correlation criteria within a region around the zero-time offset of the correlation function, defined as an interference Free Window (IFW).

FIELD OF THE INVENTION

The invention generally relates to digital transmission. In particular,it relates to a method of transmitting data using multi-carrierCode-Division Multiple Access (CDMA) for accessing a transmission systemand to a method of receiving such transmitted data.

The invention also relates to a system, a transmitter and a receiver forcarrying out the methods mentioned above.

It also relates to computer program products for carrying out suchmethods.

The invention applies particularly to future generation high data ratemobile communications systems (beyond 3^(rd) Generation).

BACKGROUND ART

Due to the increasing demand for higher rate mobile data communications,partly because multimedia traffic is expected to dominate voice trafficin the near future, the next generation of cellular wireless systems,also called 4G systems, have the important challenge of providinghigh-capacity spectrum-efficient services to the customers. Therefore,even before the fall commercial deployment of 3G systems, studies anddiscussions on 4G systems (or IMT-2010+ systems) have already started.Efforts are being made to develop an air interface that supports therequirements of the increasing mobile data traffic.

Wideband Code-Division Multiple Access (CDMA) systems have recently beenproposed for wireless communication networks. These systems providehigher capacity and higher data rates than conventional accesstechniques. Moreover, they are able to cope with the asynchronous natureof multimedia data traffic and to combat the hostile channel frequencyselectivity. However, the large frequency bandwidth of such high-speedwireless links makes them susceptible to Intersymbol Interference (ISI).Therefore, a number of multi-carrier CDMA techniques have been suggestedto improve performance over frequency selective channels. On the otherhand, one of the ways to increase the user data rate in the accessnetwork is to use a multi-carrier multiplexing technique known asOrthogonal Frequency-Division Mutiplexing (OFDM). OFDM is a goodsolution to transmit high data rates in a mobile environment, even in ahighly hostile radio channel. Multi-carrier CDMA (OFDM-CDMA) combinesOFDM and CDMA techniques. It allows to benefit from the robustnessagainst channel dispersivity of OFDM and from the high multiple accesscapacity of CDMA. Spreading is performed either in the frequency domain,leading to Multi-Carrier CDMA (MC-CDMA), or in the time domain, leadingto Multi-Tone CDMA (MT-CDMA) and Multi-Carrier Direct Sequence CDMA(MC-DS-CDMA).

OFDM techniques suffer from various drawbacks: synchronization isdifficult to perform and systems are sensitive to frequency offset andnon-linear amplification resulting in high peak-to-average power ratio(PAPR). Though multi-carrier CDMA suffers from the same drawbacks, itsmajor advantage is to lower the symbol rate in each sub-carrier allowinglonger symbol duration and hence easier channel estimation.

The article, denoted [1], by L. Vandendorpe: “Multitone spread spectrummultiple access communications system in a multipath Rician fadingchannel,” published in IEEE Transactions on Vehicular Technology, vol.44, no. 2, pages 327-337, May 1995, describes, for the uplink, theasynchronous Multi-Tone CDMA (MT-CDMA) technique as a promisingcandidate for future 4G systems. The main idea behind the structure ofMT-CDMA is to be able to increase the spreading sequence length by theaddition of multiple carriers without increasing the bandwidth, thushaving the advantage of increasing the user capacity by decreasing theMultiple Access Interference (MAI). However, this advantage is achievedat the expense of an increase in Inter-Carrier Interference (ICI) whichcounterbalances the advantage, and thus, the increase in user capacitycan be lost. Therefore, MT-CDMA systems require interferencecancellation/reduction techniques that can have prohibitive complexitiesin high data rate wireless applications.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a system, which is lesscomplex to implement than the one described in the cited article [1] andwhich yields a better quality.

The invention takes the following aspects into consideration. Large AreaSynchronized-CDMA (LAS-CDMA) has recently been proposed to enhance 3Gand 4G wireless systems and described in the document, denoted [2]:“Physical layer specification for LAS-2000,” in China WirelessTelecommunication Standards (CWTS), WG1, SWG2#4, LAS-CDMA Sub-WorkingGroup, Apr. 25^(th) 2001. LAS-CDMA uses an efficient set of spreadingcodes, called LAS codes that have perfect autocorrelation andcross-correlation properties within a region around the origin definedas the Interference-Free Window (IFW). LAS codes are described in thearticle, denoted [3], by D. Li “A high spectrum efficient multipleaccess code,” published in Proceedings of the Fifth Asia-PacificConference on Communications and Fourth Optoelectronics andCommunications Conference (APCC/OECC'99), vol. 1, pages 598-605, 1999,as a new class of high spectrum efficient multiple access codes. Similarsequences also exist in literature, such as e.g. Zero Correlation Zone(ZCZ) sequences described in the articles by P. Z. Fan, N. Suehiro andX. M. Deng “A class of binary sequences with zero correlation zone,”published in Electronics Letters, vol. 35, pages 777-779, 1999, denoted[4], or in the article by X. M. Deng and P. Z. Fan:“Spreading sequencesets with zero correlation zone,” in Electronics Letters, vol. 36, no.12, pages 982-983, December 2000, denoted [5], Low Correlation Zone(LCZ) sequences described in the article by X. H. Tang, P. Z. Fan and S.Matsufuji: “Lower bounds on the maximum correlation of sequence set withlow or zero correlation zone,” published in Electronics Letters, vol.36, no. 6, pages 551-552, March 2000, denoted [6], or the article by B.Long, P. Zhang and J. Hu:“ A generalized QS-CDMA system and the designof new spreading codes,” in IEEE Transactions on Vehicular Technology,vol. 47, pages 1267-1275, November 1998, denoted [7], and GeneralizedOrthogonal Sequences described in the article by P. Fan and L. Hao:“Generalized orthogonal sequences and their applications in synchronousCDMA systems,” in IEICE Trans. Fundamentals, vol. E83-A, no. 11, pages2054-2069, November 2000, denoted [8]. The common feature in thesesequences is that the autocorrelation and cross-correlation propertiessatisfy the desired conditions only within a certain region centered onthe origin. By using such sequences for spreading purposes in CDMA-basedsystems, it is then possible to obtain significant reductions in boththe Intersymbol Interference (ISI) if the channel delay spread issmaller than the length of the ZCZ/LCZ, and the MM if thesynchronization among users can be controlled to a permissible timedifference that takes into account the length of the LCZ/ZCZ. For LAScodes, it has been shown that the product of the number of availablecodes by the length of the IFW is directly proportional to the sequencelength. Thus, by having a longer sequence length, the number ofavailable codes and/or the length of the IFW can be increased.

The invention proposes a new system, which can use one of the spreadingsequence families mentioned above with the MT-CDMA structure. Using theinterference rejection properties of these codes allows benefiting fromthe advantages of MT-CDMA without having to suffer from ICI. By usingthe possibility of increasing the spreading sequence length withoutbandwidth expansion provided by MT-CDMA, the number of availablespreading codes and/or the length of the IFW can be increased. It isespecially relevant to increase the length of the IFW because of theincreasing channel length for high data rate wireless applications.Thus, the new system can be seen as a symbiosis where the two componentsystems enhance the relative performance of each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and additional features, which may be optionally used toimplement the invention to advantage, are apparent from and will beelucidated with reference to the drawings described hereinafter andwherein:

FIG. 1 is a conceptual block diagram illustrating an example of anMT-CDMA transmitter,

FIG. 2 is a schematic illustrating the spectrum of an MT-CDMA signal,

FIG. 3 is a conceptual block diagram illustrating an example of anMT-CDMA receiver,

FIG. 4 and FIG. 5 are schematics for illustrating the construction of anexample of a spreading code, which can be used in the invention,

FIG. 6 and FIG. 7 are graphs illustrating simulation results in a systemin accordance with the invention,

FIG. 8 is a conceptual block diagram illustrating an example of a systemin accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an MT-CDMA transmitter. The MT-CDMA scheme is mainlyproposed for the uplink communications of a cellular system due to itsasynchronous structure. An encoder ENCOD encodes incoming data symbols Sfor an arbitrary user k into encoded data symbols Sc. Aserial-to-parallel converter S/P converts the incoming encoded datasymbols Sc into Nc low rate parallel sub-streams, each of whichmodulates a sub-carrier f_(p)(p=0, . . . Nc−1), and are summed up toyield an OFDM symbol. The incoming encoded data symbol duration Ts isincreased by the factor Nc to yield T=Nc×Ts as the OFDM symbol durationat the output of the adder. The OFDM symbol is then spread by theassociated spreading waveform of user k, c_(k)(t), and transmitted.

FIG. 2 shows the spectrum of an MT-CDMA signal comprising Ncsub-carriers denoted f₀, f₁, . . . , f_(Nc−1). The sub-carrier spacingis 1/T, so the Nc parallel data sub-streams fulfill the orthogonalityrequirements before spreading. However, after spreading, the spectrum ofeach sub-carrier no longer satisfies the orthogonality condition,resulting in a major drawback of MT-CDMA systems: the Inter CarrierInterference (ICI), as illustrated by FIG. 2.

On the other hand, the tight sub-carrier spacing enables using longerspreading codes of length L, that is longer by a factor of Nc than thelength of a conventional DS-CDMA scheme, making the processing gain ofan MT-CDMA system being equal to L/Nc, which is a major advantage of thesystem. Therefore the trade-off in an MT-CDMA system is that, at theexpense of higher ICI, the system benefits from the advantages of longerspreading sequences (like the reduction in MAI and ISI due to bettercorrelation properties, having more available sequences, etc.). In achannel where these advantages are dominant, the MT-CDMA scheme canoutperform the conventional DS-CDMA scheme.

FIG. 3 shows an MT-CDMA receiver. It comprises a RAKE demodulator 30, anequalizer, which also performs interference cancellation, denoted EQ/IC,a decoder DECOD and a detector DETECT. The receiver receives a signalformed by the MT-CDMA data sequences transmitted by the transmitterdepicted in FIG. 1. The multi-carrier MT-CDMA signal, denoted r(t), isreceived by the RAKE demodulator 30. It comprises several sub-carriersignals distributed among Nc sub-carriers, denoted f₀ to f_(Nc−1), andeach sub-carrier signal having several paths called multi-paths. TheRAKE demodulator first separates the sub-carriers to demodulate thereceived signal, i.e. to perform the reverse operation to the classicalOFDM modulation. To this end, parallel multipliers multiply the receivedsignal r(t) by the sub-carriers f₀ to f_(N−1). Then, Nc RAKE combiners,denoted RAKE 0 to RAKE Nc−1, perform matched filtering on all receivedpaths, and combine them optimally by Maximum Ratio Combining. Eachbranch in the RAKE demodulator 30 of the receiver front-end can beregarded as a standard CDMA RAKE combiner tuned to the associatedsub-carrier. A parallel-to-serial converter P/S converts the paralleloutputs of the RAKE combiners into serial sequences. The serialsequences are then equalized and residual interference is cancelled withthe equalization/interference cancellation block EQ/IC. Then thesequences are decoded by the decoder DECOD which performs a reverseoperation to the encoder ENCOD depicted in FIG. 1. Then the detectorDETECT decides with an estimation of the received signal to retrieve theoriginal data S.

Since the performance of the RAKE receiver is interference-limited(determined by the correlation properties of the spreading sequenceset), post-RAKE processing in terms of equalization (EQ), InterferenceCancellation (IC) and/or Multi-User Detection (MUD) is usually necessaryfor satisfactory performance. For high data rate wireless applicationsof the next generation cellular mobile systems, this necessity can bringcomplexity problems. Furthermore, it has been shown that the overalldigital low-pass equivalent structure between the serial to parallelconverted coded symbols and the samples at the output of the RAKEcombiners conveys a Multiple Input Multiple Output (MIMO) structure.Therefore, the post-RAKE processing also has a MIMO structure, whichfurther makes it prone to complexity problems.

The low-pass equivalent transmitted signal x_(k)(t) at the output of thetransmitter of FIG. 1 is given by: $\begin{matrix}{{x_{k}(t)} = {\sqrt{\frac{P}{N_{C}}}{\sum\limits_{q = 0}^{N_{c} - 1}{\sum\limits_{n = {- \infty}}^{\infty}{{I_{k}^{q}\lbrack m\rbrack}{c_{k}(t)}{u\left( {t - {nT}} \right)}{\exp\left( {j\quad\frac{2\pi}{T}{qt}} \right)}}}}}} & (1)\end{matrix}$where P is the transmit power of all users, I_(k) ^(q)[m] is the complexsymbol on sub-carrier q of user k at instant m, c_(k)(t) is thespreading waveform of user k, and u(t) is the OFDM pulse shape which isassumed to be rectangular with unit amplitude and duration T. The RFfrequency associated with sub-carrier q is f_(q)=f₀+q/T where f₀ is somebase frequency.

Assuming a linear time-invariant channel of user k with low-pass impulseresponse g_(k)(t), the received low-pass equivalent signal r(t) in asystem with K users can be expressed as: $\begin{matrix}{{r(t)} = {{\sqrt{\frac{P}{N_{C}}}{\sum\limits_{k = 1}^{K}{\sum\limits_{q = 0}^{N_{c} - 1}{\sum\limits_{n = {- \infty}}^{\infty}{{I_{k}^{q}\lbrack m\rbrack}{h_{k}^{q}\left( {t - {mT}} \right)}}}}}} + {n(t)}}} & (2)\end{matrix}$where h_(k) ^(q)(t)=[c_(k)(t) u(t) expo 2π/Tx qt)]*g_(k)(t), * denoteslinear convolution and n(t) is the zero-mean Additive White GaussianNoise (AWGN) with two-sided power spectral density N₀. The receiver ofuser u employs a RAKE front-end with Maximal Ratio Combining (MRC) whoseoutputs are obtained by: $\begin{matrix}{{y_{u}^{p}\lbrack n\rbrack} = {\frac{1}{T\sqrt{P}}{\int_{- \infty}^{+ \infty}{{{r(t)}\left\lbrack {h_{u}^{p}\left( {t - {nT}} \right)} \right\rbrack}*\quad{\mathbb{d}t}}}}} & (3)\end{matrix}$where y_(u) ^(p)[n] is the RAKE-MRC output of user u associated withsub-carrier p at time instant n, and (.)* denotes complex conjugate.Elaborating more on the RAKE-MRC outputs, we obtain: $\begin{matrix}{{y_{u}^{p}\lbrack n\rbrack} = {{\frac{1}{\sqrt{N_{c}}}\begin{Bmatrix}{{{I_{u}^{p}\lbrack n\rbrack}{\chi_{uu}^{pp}\lbrack 0\rbrack}} + {\sum\limits_{\substack{\eta = {- \infty} \\ ({\eta \neq 0})}}^{\infty}{{I_{u}^{p}\left\lbrack {n - \eta} \right\rbrack}{\chi_{uu}^{pp}\lbrack\eta\rbrack}}} +} \\{{\sum\limits_{\substack{q = 0 \\ ({q \neq p})}}^{N_{c} - 1}{\sum\limits_{\eta = {- \infty}}^{\infty}{{I_{u}^{p}\left\lbrack {n - \eta} \right\rbrack}{\chi_{uu}^{pp}\lbrack\eta\rbrack}}}} + {\sum\limits_{\substack{k = 1 \\ ({q \neq p})}}^{K}{\sum\limits_{q = 0}^{N_{c} - 1}{\sum\limits_{\eta = {- \infty}}^{\infty}{{I_{u}^{p}\left\lbrack {n - \eta} \right\rbrack}{\chi_{uu}^{pp}\lbrack\eta\rbrack}}}}}}\end{Bmatrix}} + {\frac{1}{T\sqrt{P}}{v_{u}^{p}\lbrack n\rbrack}}}} & (4)\end{matrix}$where the channel correlation coefficients χ_(uk) ^(pq) are defined as:$\begin{matrix}{{\chi_{uk}^{pq}\lbrack\eta\rbrack} = {\frac{1}{T}{\int_{- \infty}^{+ \infty}{{{h_{k}^{q}(t)}\left\lbrack {h_{u}^{p}\left( {t - {\eta\quad T}} \right)} \right\rbrack}*\quad{\mathbb{d}t}}}}} & (5)\end{matrix}$and ν_(u) ^(p)[n] is the zero-mean AWGN sample with variance N₀Tχ_(uu)^(PP)[0]. The first term in equation (4) is the desired signal term, thesecond is the ISI term, the third is the ICI term, and the fourth is theMAI term. In all these interference terms, only L_(c) components of eachsummation are significant. L_(c) is called the channel depth and isgiven by L_(c)=└1+T_(m)/T┘ where T_(m) is the multi-path delay spread ofthe channel and where └.┘ means rounding to the closest smaller integer.Considering the generic multi-path channel model of user k as:$\begin{matrix}{{g_{k}(t)} = {\sum\limits_{l = 0}^{L_{c} - 1}{g_{k,l}{\delta\left( {t - \tau_{k,l}} \right)}}}} & (6)\end{matrix}$where {g_(k,l)} and {τ_(k,l)} denote the complex path coefficients andpath delays, respectively, equation (5) can be rewritten as:$\begin{matrix}{{\chi_{uk}^{pq}\lbrack\eta\rbrack} = {\frac{1}{T}{\sum\limits_{t = 0}^{{Lc} - 1}{\sum\limits_{i = 0}^{{Lc} - 1}{g_{k,l}g_{u,l}^{*}{\exp\left( {{j2}\quad\frac{\pi}{T}\left( {{p\quad\tau_{u,t}} - {q\quad\tau_{k,i}}} \right)} \right)}{R_{uk}^{pq}\lbrack\eta\rbrack}}}}}} & (7)\end{matrix}$where the correlation coefficients R_(uk) ^(pq)[η] are given by:$\begin{matrix}{{R_{uk}^{pq}\lbrack\eta\rbrack} = {\int_{- \infty}^{\infty}{{c_{u}^{*}\left( {t - \tau_{u,l}} \right)}{c_{k}\left( {t - \tau_{k,i}} \right)}{u\left( {t - {\eta\quad T} - \tau_{u,l}} \right)}{u\left( {t - \tau_{k,i}} \right)}{\exp\left\lbrack {{j2}\quad\frac{\pi}{T}\left( {q - p} \right)t} \right\rbrack}\quad{\mathbb{d}t}}}} & (8)\end{matrix}$

The correlation coefficients depend on the partial correlationproperties of the spreading sequences. As observed from the aboveequations, MT-CDMA trades off the reduction in correlation values due toutilization of longer spreading codes by the extra interference comingfrom the introduction of more sub-carriers.

CDMA systems with single user detection are interference-limited. Theinterference in CDMA systems is determined by the autocorrelation andcross-correlation properties of the spreading codes. An ideal code sethas no side lobes in their aperiodic/partial autocorrelations (zerooff-peak autocorrelation) and cross-correlations (zerocross-correlation) as described in [3]. However, having idealautocorrelation and cross-correlation properties are contradictinggoals, and no such code set exists. Fortunately, in order to rejectinterference, it is not necessary to have zero off-peak autocorrelationand zero cross-correlation everywhere, but within a certain regionaround the origin whose length depends on the channel delay spread,which is defined as a time length corresponding to an estimate of adifference between the time lengths of at least two differentmulti-paths, generally the longest and the shortest. So, as long assynchronism can be established that takes into account the channel delayspread, a CDMA system using such spreading codes does not suffer frominterference. Spreading code sets satisfying these properties (alsocalled generalized orthogonality conditions) exist in literature [4] to[8].

FIG. 4 and FIG. 5 show the construction of an example of these codes,denoted LAS code, which has the desired interference rejectionproperties. These codes were recently used in a new CDMA scheme calledLAS-CDMA that has been proposed for the 3G standardization process inChina, and also as a basis for 4G systems. LAS-CDMA uses this specificset of spreading codes, called LAS codes, whose off-peak partialautocorrelation and partial cross-correlation values are zero within aregion around the origin [-d,d]: the Interference Free Window, asdescribed in [3]. In order to achieve these partial autocorrelation andcross-correlation properties, zero gaps are inserted in the sequence.LAS codes are the combination of the pulse-suppressing bipolar LS codes,and the LA pulses that determine the lengths and the places of the zerogaps. Between two LA pulses, there is an LS code that comprises a Csection C_(k) and an S section S_(k) followed by an C gap and an S gap,respectively, as shown in FIG. 4. LA pulses are represented in FIG. 4 byhatched blocks inserted between the LS blocks. Hatched blocks in theframe illustrating the details of an LS symbol represent S and C gaps,respectively.

FIG. 5 shows the iterative construction of the C and S sections, whichare bipolar sequences where L′ is the length of the LS sequence withoutthe zero gap (i.e. the sum of the lengths of C_(k) and S_(k)). As anexample for a LAS code: c1=++, c2=+−, s1=−+ and s2=−−at the first levelwhere L′=4. As for the LA codes, they are used to identify acell/sector, and different LA codes are obtained by permuting the basicLA code whose pulse positions are depicted in table 1 below. TABLE 1Primary LA code pulse position LA gap 0 2 4 6 8 10 12 14 16 18 20 22 2426 28 36 1 LA pulse 136 274 414 556 700 846 994 1144 1296 1450 1606 17641924 2086 2250 2422 2259 position

The construction of a LAS code shown in FIG. 4 is an example, whichcorresponds to the Chinese 3G standard specification proposal [2]. The Cand S sections of the LS code are of length 64 (forming an LS code oflength L′=128), the length of the C and S gaps is 4, the number of LApulses is 17, and the total number of chips found in the LAS code is2559. With these parameters, the constructed code has an IFW of length9, i.e. d=4. Further details on the construction of LAS codes are givenin the document [2].

LAS codes have certain drawbacks: the insertion of zeros in the sequencecauses a loss in spectral efficiency, and the number of sequencessatisfying the generalized orthogonality conditions is limited. It hasbeen shown that the upper bound on the number of such availablesequences is given by L′/(d+1). So, in order to increase the number ofavailable sequences, the sequence length would have to be increased,which would result in bandwidth expansion and/or the IFW size would haveto be decreased, which would result in an increase of interference.

Using LAS-CDMA in MT-CDMA leads to a new system denoted LAS-MT-CDMA, inaccordance with the invention. This new system brings with it asymbiosis, which benefits from the advantages of both systems withoutsuffering from all the drawbacks. In other words, the advantages of onesystem help to overcome the drawbacks of the other, and vice versa. Byusing LAS codes in MT-CDMA systems, the impact of ICI, ISI and MAI onsystem performance can be decreased. Considering equations (4), (7) and(8), the weight of the interference terms in the RAKE-MRC outputs willdecrease due to the decrease in the correlation coefficients.

FIG. 6 and FIG. 7 show computer simulation results in order to be ableto see the respective effects of increasing the number of sub-carriersin MT-CDMA and in LAS-MT-CDMA. FIG. 6 depicts the simulation results ofboth systems for one user with an increasing number of sub-carriers:Nc=1, Nc=2 and Nc=4. The curves represent the bit error rate BER withrespect to the energy per bit over spectral density of noise Eb/No.MT-CDMA system employs extended Gold sequences. In all simulations, astatic 2-tap EQ channel with a delay spread of 2Tc is used. Themodulation scheme is QPSK. In order to keep the bandwidth equal forNc=1, Nc=2 and Nc=4, spreading sequences of length 128, 256 and 512 areused respectively. The receiver consists of a two-finger RAKE receiverwith MRC followed by a hard decision device. There is no equalizer, nointerference canceller and no coding. Perfect channel state informationis assumed. For comparison purposes, the performance on AWGN channel isalso depicted in the same figure.

It can be observed from the simulation results that the MT-CDMA schemesuffers from extra interference with the addition of more sub-carriers.It means that the correlation properties of the extended Gold sequencescannot overcome the detrimental effects of the additional ICI introducedby the sub-carriers. However, this is not the case for LAS-MT-CDMA,wherein the addition of more sub-carriers does not introduce additionalICI thanks to the IFW (whose length is greater than the channel delayspread), so the performance degradation is avoided. It can also beobserved that the performance of LAS-MT-CDMA on a 2-tap EQ channel isthe same with the AWGN channel, which proves the efficiency of LAScodes. By looking at the correlation properties of LAS codes it can besaid that even if the length of the IFW is smaller than the channeldelay spread, the amount of introduced interference is still smallercompared to MT-CDMA.

The performance of LAS-MT-CDMA scheme with two different numbers ofusers is illustrated in FIG. 7. It gives comparative simulation resultsof MT-CDMA and LAS-MT-CDMA with a single user K=1 and with two usersK=2. In this second set of simulations, the same channel and systemmodels, modulation scheme and receiver structure are employed. Thesynchronism between different users stays within 2Tc. As can be observedfrom the Figure, addition of more users causes performance degradationin MT-CDMA due to the imperfect correlation properties of the extendedGold sequences. However, this is not the case in LAS-MT-CDMA, as long assynchronism between users can be maintained to a certain extent thattakes into account the channel delay spread and the length of the IFW,addition of more users does not introduce MAI and the system performancedoes not degrade in LAS-MT-CDMA. With these simulation parameters, thenumber of users can be increased to 16 (the number of availablesequences) without introducing MAI. This means that it is possible toavoid the “near-far effect” without having to implement the relativelycomplex MUD algorithms. Note that, due to the insertion of zeros,LAS-MT-CDMA has lower spectral efficiency than MT-CDMA (about 17%). Inorder to compare the two systems with the same spectral efficiency,coding can be introduced.

LAS-MT-CDMA is also advantageous when compared to LAS-CDMA. By usingmultiple sub-carriers in a LAS-CDMA system, the number of availablesequences and/or the IFW size (both of which have the effect ofincreasing system capacity) can be increased by increasing the sequencelength without bandwidth expansion. Increasing the IFW size isespecially important when considering the longer channel length due tohigh data rates in wireless channels. For example, the LAS-CDMAspecification uses a single carrier (Nc=1) with a code of length L′=128.With an IFW of d=4, the number of available sequences is 16. If we usetwo carriers (Nc=2), keeping the same user data rate and transmissionbandwidth, we can use sequences of length L′=256 in the LAS-MT-CDMAscheme. With the same IFW as before (d=4), the number of availablesequences can be increased up to 32. This means a twofold capacityincrease, because the performance of the two systems is the same due tothe total interference rejection capability of LAS codes. Alternatively,keeping the number of available sequences at 16, it is possible todesign LAS codes having an IFW with d=8. This means that the system cansupport twice the data rate that can be supported by LAS-CDMA. Since themeaningful figure of merit for a multiple access system is its totalspectral efficiency that is defined in terms of the total datathroughput per sector per system bandwidth, increasing the average datarate twice for all users means doubling the spectral efficiency.Considering the demands of 4G systems in terms of spectral efficiency,this improvement is especially significant.

FIG. 8 shows a system in accordance with the invention, comprising atransmitter 81, a receiver 82 and a transmission channel 83 fortransmitting data from the transmitter to the receiver. In a mobilecommunication system, for example, the user equipment would be thereceiver and the base station the transmitter during a downlinktransmission, whereas in an uplink transmission, the base station wouldbe the receiver and the user equipment the transmitter. The transmitteris similar in design to the MT-CDMA transmitter depicted in FIG. 1,except that the spreading codes used have specific interferencerejecting properties (e.g. LAS codes) as defined with reference to FIGS.4 and 5, i.e. they satisfy predetermined auto-correlation and/orcross-correlation criteria within a region around the origin, defined asan Interference-Free Window (IFW). The data to be transmitted aremodulated using Orthogonal Frequency-Division Multiplexing (OFDM) beforebeing spread with these specific codes. The receiver is similar indesign to the one depicted in FIG. 3, except the received sequences arespread by one of the spreading codes mentioned.

In conclusion, a new system has been described, which advantageouslybenefits from the interference rejection properties of judiciouslyselected spreading codes with the capability of MT-CDMA to increase thespreading sequence length without expanding the bandwidth. Moreover,this allows enhancing and extending the advantages brought by bothsystems without suffering from their drawbacks. The interferenceintroduced by multiple sub-carriers are rejected by the selectedspreading codes and, with the addition of multiple sub-carriers,increasing the sequence length can increase the efficiency of thespreading codes. Simulation results have shown that addition of multiplesub-carriers and users does not deteriorate the system performance, andleads to a capacity increase. Last but not least, the loss in spectralefficiency of certain of these spreading codes, notably the LAS codes,due to the insertion of zero gaps, which is a drawback, can be overcomeby the use of other similar sequences in literature that do not requireinsertion of zero gaps e.g. ZCZ/LCZ sequences. It is also possible tocompensate for this loss by appropriate channel coding.

The drawings and their descriptions hereinbefore illustrate rather thanlimit the invention. It will be evident that there are numerousalternatives, which fall within the scope of the appended claims. Inthis respect, the following closing remarks are made.

There are numerous ways of implementing functions by means of items ofhardware or software, or both. In this respect, the drawings are verydiagrammatic, each representing only one possible embodiment of theinvention. Thus, although a drawing shows different functions asdifferent blocks, this by no means excludes that a single item ofhardware or software carries out several functions. Nor does it excludethat an assembly of items of hardware or software, or both carries out afunction.

Any reference sign in a claim should not be construed as limiting theclaim. Use of the verb “to comprise” and its conjugations does notexclude the presence of elements or steps other than those stated in aclaim. Use of the article “a” or “an” preceding an element or step doesnot exclude the presence of a plurality of such elements or steps.Documents referred to:

-   [1]: L. Vandendorpe: “Multitone spread spectrum multiple access    communications system in a multipath Rician fading channel,”    published in IEEE Transactions on Vehicular Technology, vol. 44, no.    2, pages 327-337, May 1995.-   [2]: “Physical layer specification for LAS-2000,” in China Wireless    Telecommunication Standards (CWTS), WG1, SWG2#4, LAS-CDMA    Sub-Working Group, Apr. 25^(th) 2001.-   [3]: D. Li:“A high spectrum efficient multiple access code,”    published in Proceedings of the Fifth Asia-Pacific Conference on    Communications and Fourth Optoelectronics and Communications    Conference (APCC/OECC'99), vol. 1, pages 598-605, 1999.-   [4]: P. Z. Fan, N. Suehiro and X. M. Deng:“A class of binary    sequences with zero correlation zone,” published in Electronics    Letters, vol. 35, pages 777-779, 1999. [5]: X. M. Deng and P. Z.    Fan:“Spreading sequence sets with zero correlation zone,” in    Electronics Letters, vol. 36, no. 12, pp. 982-983, December 2000.-   [6]: X. H. Tang, P. Z. Fan and S. Matsufuji: “Lower bounds on the    maximum correlation of sequence set with low or zero correlation    zone,” published in Electronics Letters, vol. 36, no. 6, pages    551-552, March 2000.-   [7]: B. Long, P. Zhang and J. Hu:“ A generalized QS-CDMA system and    the design of new spreading codes,” in IEEE Transactions on    Vehicular Technology, vol. 47, pages 1267-1275, November 1998.-   [8]: P. Fan and L. Hao: “Generalized orthogonal sequences and their    applications in synchronous CDMA systems,” in IEICE Trans.    Fundamentals, vol. E83-A, no. 11, pages 2054-2069, November 2000.

1. A method of transmitting data using multi-carrier Code-DivisionMultiple Access (CDMA) for accessing a transmission system, the methodcomprising a step of modulating the data to be transmitted usingOrthogonal Frequency-Division Multiplexing (OFDM) for producing OFDMmodulated data symbols and a step of spreading the OFDM modulated datasymbols with spreading codes including a set of predefined sequenceswherein the sequences are predefined so that they satisfy predeterminedauto-correlation and/or cross-correlation criteria within a regionaround the origin, defined as an Interference-Free Window (IFW).
 2. Amethod as claimed in claim 1, wherein the transmission system comprisesa transmitter, a receiver and a transmission channel, for transmittingthe data from the transmitter to the receiver via the transmissionchannel, the transmission channel including a set of multi-paths withassociated time lengths, the transmission channel having a channel delayspread defined as a time length corresponding to an estimate of adifference between the time lengths of at least two differentmulti-paths, the length of the Interference-Free Window (IFW) dependingon the channel delay spread.
 3. A method as claimed in claim 1, whereinthe sequences are such that their off-peak partial autocorrelation andpartial cross-correlation values are zero within the Interference-FreeWindow (IFW).
 4. A method as claimed in claim 2, wherein the sequencesare such that they comprise zero gaps.
 5. A transmitter for transmittingdata using multi-carrier Code-Division Multiple Access (CDMA) foraccessing a transmission system, comprising a modulator for modulatingthe data to be transmitted using Orthogonal Frequency-DivisionMultiplexing (OFDM) for producing OFDM modulated data symbols and amixer for spreading the OFDM modulated data symbols with spreading codesincluding a set of predefined sequences, wherein the sequences arepredefined so that they satisfy predetermined auto-correlation and/orcross-correlation criteria within a region around the origin, defined asan Interference-Free Window (IFW).
 6. A method of receivingmulti-carrier data sequences transmitted via a transmission system usingmulti-carrier Code-Division Multiple Access (CDMA) for accessing thetransmission system, the data sequences being OFDM modulated beforebeing spread with a set of predefined sequences satisfying predeterminedauto-correlation and/or cross-correlation criteria within a regionaround the origin, defined as an Interference Free Window (IFW), themethod comprising a step of demodulating the received multi-carrier datasequences with respect to a predefined set of sub-carriers and to theset of predefined data sequences.
 7. A receiver for receiving datasequences transmitted via a transmission system using multi-carrierCode-Division Multiple Access (CDMA) for accessing the transmissionsystem, the data sequences being OFDM modulated before being spread witha set of predefined sequences satisfying predetermined auto-correlationand/or cross-correlation criteria within a region around the origin,defined as an Interference-Free Window (IFW), the receiver comprising aset of rake combiners tuned to associated sub-carriers for demodulatingthe received data sequences.
 8. A computer program product for atransmitter computing a set of instructions, which when loaded in thereceiver, causes the receiver to carry out the method as claimed inclaim
 1. 9. A computer program product for a receiver computing a set ofinstructions, which when loaded in the receiver, causes the receiver tocarry out the method as claimed in claim
 6. 10. A system comprising atleast a transmitter and a receiver for transmitting data from thetransmitter to the receiver using multi-carrier Code-Division MultipleAccess (CDMA) for enabling the transmitter to access the transmissionsystem, the data to be transmitted being modulated using OrthogonalFrequency-Division Multiplexing (OFDM) before being spread with a set ofpredefined sequences wherein the sequences are predefined so that theysatisfy predetermined auto-correlation and/or cross-correlation criteriawithin a region around the origin, defined as an Interference-FreeWindow (IFW).